1. Field of the Invention
This invention relates to an outboard motor exhaust system.
2. Description of the Related Art
In outboard motors incorporating an internal combustion engine used as a power source for driving a propeller, the exhaust gas generated by the engine is generally passed through the boss portion of the propeller to be discharged rearward into the water. However, when engine exhaust gas is discharged into the water rearward of the propeller, it is drawn in by the propeller when the shift position is reverse and the boat moves rearward. This is disadvantageous because it decreases thrust.
In order to solve this problem, Japanese Laid-Open Patent Application No. Hei 7(1995)-144693 teaches a configuration which during reverse boat travel discharges the exhaust gas into the atmosphere (outside air) through an exhaust gas passage provided above the water level of the outboard motor. The exhaust gas passage is provided midway with an exhaust valve mechanically linked with the outboard motor shift mechanism. When the shift mechanism establishes the reverse gear, the exhaust valve is opened via the linkage.
In the conventional outboard motor, shift position is changed by the operator manually operating a shift lever mechanically linked with the shift mechanism. Therefore, the configuration of '693, which interlocks the exhaust valve opening operation with the shift mechanism operation, has a problem in that it increases the manipulation load of the shift lever, thereby degrading the shift feel.